Conventionally, gas barrier plastic film, which has a plastic film support and, provided on a surface of the plastic film support, an inorganic thin film formed of a material such as silicon oxide, aluminum oxide, or magnesium oxide, is widely employed as a wrapping material for articles which must be isolated from gases such as water vapor and oxygen; e.g., a wrapping material for preventing deterioration of food, industrial materials, drugs, etc. In addition to the wrapping use, in recent years, such gas barrier plastic films have been envisaged to find new uses as a transparent conductive sheet for use in devices such as liquid crystal displays, solar cells, electromagnetic shields, touch panels, EL substrates, and color filters.
Various improvements have been investigated with respect to the above gas barrier plastic films each having an inorganic thin film provided on a support (hereinafter may be referred to as “inorganic thin film-attached gas barrier films”), from the viewpoint of prevention of deterioration in gas barrier performance. Among the improvements, in one known approach, a coating layer formed of polyurethane, polyester, or a mixture of polyurethane and polyester is provided on a surface of the inorganic thin film (see, for example, .[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2-50837. Other approaches have also been known; for example, a gas barrier film in which a mixture of a water-soluble polymer and a metal alkoxide is applied onto a surface of the inorganic thin film (see, for example, [Patent Document 2] Japanese Patent Application Laid-Open (kokai) No. 8-267637 a gas barrier film in which a mixture of a water-soluble polymer and inorganic particles is applied onto a surface of the inorganic thin film (see, for example,) [Patent Document 3] Japanese Patent Application Laid-Open (kokai) No. 11-151786, and a gas barrier film in which a gas barrier resin such as a vinylidene chloride copolymer or an ethylene-vinyl alcohol copolymer is applied onto a surface of the inorganic thin film (see, for example, [Patent Document 4] Japanese Patent Application Laid-Open (kokai) No. 7-80986.
However, the above approaches are merely means for preventing deterioration in gas barrier performance which the inorganic thin film-attached gas barrier film per se exhibits upon use, such as secondary processing. Hitherto, there have never been disclosed means for improving the intrinsic gas barrier performance of a film having an inorganic thin film.
Meanwhile, gas barrier performance of inorganic thin film-attached gas barrier film is generally improved after production thereof, since conditions of the inorganic thin film are stabilized with elapse of time. Thus, gas barrier performance of the film immediately after production thereof is to be further improved. One disclosed means for enhancing gas barrier performance includes providing water to silicon oxide thin film and heating the film (see, for example, [Patent Document 5] Japanese Patent No. 2674827. However, this method has two steps, and the attained gas barrier performance is insufficient in relation to the characteristic level required for wrapping materials in recent years. There is also proposed an approach in which a non-electrolytic water-soluble substance such as poly(vinyl alcohol) is provided as a protective film on a surface of alumina film (see, for example, [Patent Document 6] Japanese Patent Application Laid-Open (kokai) No. 10-100301, but the water vapor barrier performance thereof is still insufficient. There are further proposed an approach in which a water vapor trapping layer containing a hygroscopic material is provided on a gas barrier layer formed of a metal oxide (see, for example, Patent Document 7] Japanese Patent Application Laid-Open (kokai) No. 10-329256 and another approach in which a gas barrier layer is formed from a layer containing a silicic acid condensate and a layer containing high-hydrogen-bond resin and an acidic substance (see, for example, [Patent Document 8] Japanese Patent Application Laid-Open (kokai) No. 2003-170522 but these gas barrier films still exhibit insufficient water vapor barrier performance.